BACKGROUND: Patients with moderate to severe rosacea often seek treatment to reduce erythema and vascular markings. Few studies have looked at the effectiveness of the novel treatment, brimonidine topical gel 0.33%, trademark name Mirvaso®, in the treatment of rosacea. We report the use of optical coherence tomography (OCT) scanning to monitor the effectiveness of Mirvaso® on in vivo skin. OCT is a non-invasive optical imaging technique that can provide high-resolution imaging of vessel and cellular morphology. OCT may be useful as a pre-treatment assessment tool for identifying possible morphologic features in the skin that may serve as outcome predictors. OCT may also serve as a monitoring tool in the treatment of rosacea. OBJECTIVE: To examine and describe how OCT skin morphology changes when exposed to brimonidine topical gel 0.33% in the treatment of erythematotelangiectatic rosacea. METHODS: Normal in vivo telangiectasias and erythematous patches and papules were examined prior to treatment clinically, dermatoscopically, and through OCT scans. Brimonidine topical gel 0.33% was applied to the face and OCT images were acquired at defined time intervals: baseline; immediately (<5 minutes) after application; 4 hours after application; and after 2 weeks' once daily application. OCT morphology was then described. RESULTS: OCT imaging showed an increase in the mean gray value (MGV), a measure of dermal reflectivity, corresponding to a decrease in dermal edema. MGV measurements for the nasal telangiectasia were: baseline, MGV 10,471 (standard deviation [SD] 6,847); immediate, MGV 15,634 (SD 8,983); after 4 hours, MGV 16,357 (SD 7,647); and after 2 weeks, MGV 15,505 (SD 6,870). MGV measurements for the chin erythema were: baseline, MGV 8,850 (SD 4,969); immediate, MGV 10,799 (SD 5,266); after 4 hours, MGV 12,419 (SD 6,714); and after 2 weeks, MGV 13,395 (SD 6,170). No significant change in vessel lumen diameter was appreciated. Vessel lumen diameter for the facial papule ranged from 0.13 mm at baseline, 0.09 mm immediately after treatment, 0.09 mm after 4 hours, and 0.11 mm after 2 weeks. CONCLUSIONS: OCT scanning showed a decrease in the dermal hyporeflectivity of the dermis consistent with a decrease in dermal edema. The OCT scans obtained did not show any significant change in vessel lumen diameter. These results may reflect an increase in vascular tone, which can be attributable to the clinical improvement and decreased erythema noted in the patient. This technology could potentially be used for the non-invasive in vivo monitoring of other topical treatments.